Issue #589: add public setter on ArrowKeyStepper to override scroll indexes

[alexandro81]

Add a public setter on ArrowKeyStepper in order to override the scroll indexes in its local state.
See issue #589 for details.

[bvaughn]

Hey @alexandro81. Thanks for creating this PR! Looks great! I wonder if I could trouble you to also update the ArrowKeyStepper.md docs to list the new public method? (Like we do for List)

[alexandro81]

Of course @bvaughn !
Done!

[bvaughn]

Thanks! This has been merged, although I made one small tweak for compatibility with existing APIs. The new method uses named parameters- rather than setScrollIndexes(scrollToRow, scrollToColumn) it's setScrollIndexes({ scrollToColumn, scrollToRow })

This change will go out in minor release 9.1.0 momentarily and I'll link to your PR in the CHANGELOG 😄